1/*
2 * pcc-cpufreq.txt - PCC interface documentation
3 *
4 * Copyright (C) 2009 Red Hat, Matthew Garrett <mjg@redhat.com>
5 * Copyright (C) 2009 Hewlett-Packard Development Company, L.P.
6 * Nagananda Chumbalkar <nagananda.chumbalkar@hp.com>
7 *
8 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
9 *
10 * This program is free software; you can redistribute it and/or modify
11 * it under the terms of the GNU General Public License as published by
12 * the Free Software Foundation; version 2 of the License.
13 *
14 * This program is distributed in the hope that it will be useful, but
15 * WITHOUT ANY WARRANTY; without even the implied warranty of
16 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or NON
17 * INFRINGEMENT. See the GNU General Public License for more details.
18 *
19 * You should have received a copy of the GNU General Public License along
20 * with this program; if not, write to the Free Software Foundation, Inc.,
21 * 675 Mass Ave, Cambridge, MA 02139, USA.
22 *
23 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
24 */
25 26 27 Processor Clocking Control Driver
28 ---------------------------------
29 30Contents:
31---------
321. Introduction
331.1 PCC interface
341.1.1 Get Average Frequency
351.1.2 Set Desired Frequency
361.2 Platforms affected
372. Driver and /sys details
382.1 scaling_available_frequencies
392.2 cpuinfo_transition_latency
402.3 cpuinfo_cur_freq
412.4 related_cpus
423. Caveats
43 441. Introduction:
45----------------
46Processor Clocking Control (PCC) is an interface between the platform
47firmware and OSPM. It is a mechanism for coordinating processor
48performance (ie: frequency) between the platform firmware and the OS.
49 50The PCC driver (pcc-cpufreq) allows OSPM to take advantage of the PCC
51interface.
52 53OS utilizes the PCC interface to inform platform firmware what frequency the
54OS wants for a logical processor. The platform firmware attempts to achieve
55the requested frequency. If the request for the target frequency could not be
56satisfied by platform firmware, then it usually means that power budget
57conditions are in place, and "power capping" is taking place.
58 591.1 PCC interface:
60------------------
61The complete PCC specification is available here:
62http://www.acpica.org/download/Processor-Clocking-Control-v1p0.pdf 63 64PCC relies on a shared memory region that provides a channel for communication
65between the OS and platform firmware. PCC also implements a "doorbell" that
66is used by the OS to inform the platform firmware that a command has been
67sent.
68 69The ACPI PCCH() method is used to discover the location of the PCC shared
70memory region. The shared memory region header contains the "command" and
71"status" interface. PCCH() also contains details on how to access the platform
72doorbell.
73 74The following commands are supported by the PCC interface:
75* Get Average Frequency
76* Set Desired Frequency
77 78The ACPI PCCP() method is implemented for each logical processor and is
79used to discover the offsets for the input and output buffers in the shared
80memory region.
81 82When PCC mode is enabled, the platform will not expose processor performance
83or throttle states (_PSS, _TSS and related ACPI objects) to OSPM. Therefore,
84the native P-state driver (such as acpi-cpufreq for Intel, powernow-k8 for
85AMD) will not load.
86 87However, OSPM remains in control of policy. The governor (eg: "ondemand")
88computes the required performance for each processor based on server workload.
89The PCC driver fills in the command interface, and the input buffer and
90communicates the request to the platform firmware. The platform firmware is
91responsible for delivering the requested performance.
92 93Each PCC command is "global" in scope and can affect all the logical CPUs in
94the system. Therefore, PCC is capable of performing "group" updates. With PCC
95the OS is capable of getting/setting the frequency of all the logical CPUs in
96the system with a single call to the BIOS.
97 981.1.1 Get Average Frequency:
99----------------------------
100This command is used by the OSPM to query the running frequency of the
101processor since the last time this command was completed. The output buffer
102indicates the average unhalted frequency of the logical processor expressed as
103a percentage of the nominal (ie: maximum) CPU frequency. The output buffer
104also signifies if the CPU frequency is limited by a power budget condition.
105 1061.1.2 Set Desired Frequency:
107----------------------------
108This command is used by the OSPM to communicate to the platform firmware the
109desired frequency for a logical processor. The output buffer is currently
110ignored by OSPM. The next invocation of "Get Average Frequency" will inform
111OSPM if the desired frequency was achieved or not.
112 1131.2 Platforms affected:
114-----------------------
115The PCC driver will load on any system where the platform firmware:
116* supports the PCC interface, and the associated PCCH() and PCCP() methods
117* assumes responsibility for managing the hardware clocking controls in order
118to deliver the requested processor performance
119 120Currently, certain HP ProLiant platforms implement the PCC interface. On those
121platforms PCC is the "default" choice.
122 123However, it is possible to disable this interface via a BIOS setting. In
124such an instance, as is also the case on platforms where the PCC interface
125is not implemented, the PCC driver will fail to load silently.
126 1272. Driver and /sys details:
128---------------------------
129When the driver loads, it merely prints the lowest and the highest CPU
130frequencies supported by the platform firmware.
131 132The PCC driver loads with a message such as:
133pcc-cpufreq: (v1.00.00) driver loaded with frequency limits: 1600 MHz, 2933
134MHz
135 136This means that the OPSM can request the CPU to run at any frequency in
137between the limits (1600 MHz, and 2933 MHz) specified in the message.
138 139Internally, there is no need for the driver to convert the "target" frequency
140to a corresponding P-state.
141 142The VERSION number for the driver will be of the format v.xy.ab.
143eg: 1.00.02
144 ----- --
145 | |
146 | -- this will increase with bug fixes/enhancements to the driver
147 |-- this is the version of the PCC specification the driver adheres to
148 149 150The following is a brief discussion on some of the fields exported via the
151/sys filesystem and how their values are affected by the PCC driver:
152 1532.1 scaling_available_frequencies:
154----------------------------------
155scaling_available_frequencies is not created in /sys. No intermediate
156frequencies need to be listed because the BIOS will try to achieve any
157frequency, within limits, requested by the governor. A frequency does not have
158to be strictly associated with a P-state.
159 1602.2 cpuinfo_transition_latency:
161-------------------------------
162The cpuinfo_transition_latency field is 0. The PCC specification does
163not include a field to expose this value currently.
164 1652.3 cpuinfo_cur_freq:
166---------------------
167A) Often cpuinfo_cur_freq will show a value different than what is declared
168in the scaling_available_frequencies or scaling_cur_freq, or scaling_max_freq.
169This is due to "turbo boost" available on recent Intel processors. If certain
170conditions are met the BIOS can achieve a slightly higher speed than requested
171by OSPM. An example:
172 173scaling_cur_freq : 2933000
174cpuinfo_cur_freq : 3196000
175 176B) There is a round-off error associated with the cpuinfo_cur_freq value.
177Since the driver obtains the current frequency as a "percentage" (%) of the
178nominal frequency from the BIOS, sometimes, the values displayed by
179scaling_cur_freq and cpuinfo_cur_freq may not match. An example:
180 181scaling_cur_freq : 1600000
182cpuinfo_cur_freq : 1583000
183 184In this example, the nominal frequency is 2933 MHz. The driver obtains the
185current frequency, cpuinfo_cur_freq, as 54% of the nominal frequency:
186 187 54% of 2933 MHz = 1583 MHz
188 189Nominal frequency is the maximum frequency of the processor, and it usually
190corresponds to the frequency of the P0 P-state.
191 1922.4 related_cpus:
193-----------------
194The related_cpus field is identical to affected_cpus.
195 196affected_cpus : 4
197related_cpus : 4
198 199Currently, the PCC driver does not evaluate _PSD. The platforms that support
200PCC do not implement SW_ALL. So OSPM doesn't need to perform any coordination
201to ensure that the same frequency is requested of all dependent CPUs.
202 2033. Caveats:
204-----------
205The "cpufreq_stats" module in its present form cannot be loaded and
206expected to work with the PCC driver. Since the "cpufreq_stats" module
207provides information wrt each P-state, it is not applicable to the PCC driver.
208